Fix to remove /wd4018 (-Wno-sign-compare) for /device/vr

This is a part of effort to remove /wd4018 warning suppression.

Master CL is
https://chromium-review.googlesource.com/c/chromium/src/+/1152755

This CL was uploaded by git cl split.

R=bajones@chromium.org

Bug: 868248, 588506
Cq-Include-Trybots: luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I980cd92d9e69cd3914e2066f159a786959823ba2
Reviewed-on: https://chromium-review.googlesource.com/1152839Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578691}

device/vr/openvr/openvr_render_loop.cc

@@ -285,7 +285,7 @@ void OpenVRRenderLoop::UpdateControllerState() {
                                                state.tracked_devices_poses,
                                                vr::k_unMaxTrackedDeviceCount);
 
-    for (int i = 0; i < vr::k_unMaxTrackedDeviceCount; ++i) {
+    for (uint32_t i = 0; i < vr::k_unMaxTrackedDeviceCount; ++i) {
       state.device_class[i] = vr_system->GetTrackedDeviceClass(i);
       if (state.device_class[i] == vr::TrackedDeviceClass_Controller) {
         state.have_controller_state[i] = vr_system->GetControllerState(
@@ -296,7 +296,7 @@ void OpenVRRenderLoop::UpdateControllerState() {
               vr_system->GetUint64TrackedDeviceProperty(
                   i, vr::Prop_SupportedButtons_Uint64);
 
-          for (int j = 0; j < vr::k_unControllerStateAxisCount; ++j) {
+          for (uint32_t j = 0; j < vr::k_unControllerStateAxisCount; ++j) {
             state.axis_type[i][j] = vr_system->GetInt32TrackedDeviceProperty(
                 i, static_cast<vr::TrackedDeviceProperty>(
                        vr::Prop_Axis0Type_Int32 + j));